Many bacteria build specialized differentiated cells in order to survive environmental insults, to disseminate in the environment, and to resist host defenses, antibiotics and disinfectants. A striking example of this type of cell differentiation by many important bacterial pathogens is the formation of endospores (e.g., Bacillus anthracis, Bacillus cereus, Clostridium difficile). Our overall goal is to understand the special molecular mechanisms that guide the development of endospores in order to identify novel targets for disrupting spore development. We will exploit the model system of endospore development in Bacillus subtilis because this system is amenable to analysis and its study has led to the discovery of many mechanisms that control gene expression in most bacteria, including pathogens. In this proposal we focus on the roles of a protein complex that connects two cells during endospore development. Early after the onset of endospore formation the cell divides asymmetrically giving rise to two dissimilar sibling cells. One of these cells (the forespore) develops into the endospore, while the other (the mother cell) becomes a terminally differentiated cell that nurtures the developing endospore. Remarkably, gene expression is coordinated between the forespore and mother cell (e.g., expression of specific genes in the mother cell is required before a subsequent set of genes is expressed in the forespore). The communication between the mother cell and forespore depends upon the targeting of several groups of proteins to the interface between the mother cell and forespore. The mechanisms for subcellular localization of specific proteins are of fundamental importance in cell biology and development. The central questions to be addressed in this proposal are how are specific proteins localized to the interface between the mother cell and forespore, and what are their functions. The two key proteins in this study, SpoIIQ and SpoIIIAH, interact through two membranes to connect the forespore and the mother cell. The SpoIIQ-SpoIIIAH complex serves as the founder that marks the membrane interface between the forespore and mother cell for the recruitment of other proteins to this subcellular location, including peptidoglycan hydrolases that function during the phagocytic-like engulfment of the forespore membrane by the mother cell membrane, proteins required for signaling from the forespore to the activate mother cell gene expression, and mother-cell proteins required for gene expression in the forespore. Moreover, the SpoIIQ-SpoIIIAH complex appears to form a new type of channel or transporter that connects the mother cell and forespore cytoplasms. The specific aims of this proposal are designed to identify which and how other proteins are recruited to this intercellular connection, and how these protein complexes function to coordinate gene expression between the two cells.